A dielectric material may be shaped to construct an optical whispering-gallery-mode (“WGM”) resonator which supports one or more whispering gallery (“WG”) modes. These WG modes represent optical fields confined in an interior region close to the surface of the resonator due to the total internal reflection at the boundary. For example, microspheres with diameters from few tens of microns to several hundreds of microns have been used to form compact optical WGM resonators. Such a spherical resonator can include at least a portion of the sphere that comprises the sphere's equator. The resonator dimension is generally much larger than the wavelength of light so that the optical loss due to the finite curvature of the resonators is small. As a result, a high quality factor, Q, may be achieved in such resonators. Some microspheres with sub-millimeter dimensions have been demonstrated to exhibit very high quality factors for light waves, ranging from 103 to 109 for quartz microspheres. Hence, optical energy, once coupled into a whispering gallery mode, can circulate within the WGM resonator with a long photon life time. Such hi-Q WGM resonators may be used in many optical applications, including optical filtering, optical delay, optical sensing, lasers, and opto-electronic oscillators.
A sphere is calculated to sustain a WGM if the internally reflected light returns in phase after a complete orbit. This depends on the spheres diameter and refractive index. An important resonator parameter is the quality factor, or Q-factor and is defined as the ratio 2Π(stored energy)/(energy lost per cycle). Extremely strong confinement of light within the spherical cavity is possible with Q-factors of up to 1010. In addition to the high photon densities in the mode volumes, there are intense evanescent fields in proximity to the outer surfaces. The utility of using a resonance tuned microsphere is the excitation photon has increased probability of interaction with a surface analyte as it orbits the sphere many thousands of cycles thus having a higher probability of generating Raman photons or infrared absorptions.
The field of Surface Enhanced Raman Spectroscopy (SERS) has grown considerably since the discovery that a Raman signal can be greatly enhanced for molecules absorbed on special metallic nano-structures and particles. The discipline has expanded to include Surface Enhanced Infrared Absorption (SEIRA) and is now more generally referred to as Surface Enhanced Spectroscopy. In conventional SERS and SEIRA, the signal enhancement is mediated by a plasmon resonance mechanism based on metallic particles. Other optical resonance mechanisms have also been used to provide localized field enhancement when special optical structures are illuminated. SEIRA has been produced using a phonon resonance mechanism with dielectric particles. These distinct resonance mechanisms may be coupled together for plasmon-phonon mediated enhancement of the signal. In other hybrid resonance systems, the plasmon based resonance has been coupled to whispering-gallery mode (WGM) resonances of micro-spheres to improve signal enhancement.
U.S. Patent Publication No. 2010/0264300 to Savchenkov et al., which is incorporated by reference herein in its entirety, is generally directed to an optical device including a whispering gallery mode (WGM) optical resonator configured to support one or more whispering gallery modes; and a photodetector optically coupled to an exterior surface of the optical resonator to receive evanescent light from the optical resonator to detect light inside the optical resonator.
Likewise, U.S. Patent Publication No. 2010/0231903 to Sumetsky, which is incorporated by reference herein in its entirety, is generally directed to an optical microresonator configured as an optical microbubble formed along a section of an optical microcapillary. The curvature of the outer surface of the microbubble creates an optical resonator with a geometry that encourages the circulating WGMs to remain confined in the central region of the bubble, creating a high Q optical resonator. The resonator may be tuned by modifying the physical properties of the microbubble, allowing the resonator to be used as an optical filter. The resonator may also be used as a sensor or laser by introducing the material to be sensed (or the active laser material) into the microcapillary along which the microbubble is formed.
L. K. Ausman and G. C. Schatz, The Journal of Chemical Physics, 129, 054704, 2008), (hereinafter “Ausman”), which is incorporated by reference herein in its entirety, report the results of calculations based on the Mie theory to determine the locally enhanced electric fields due to whispering-gallery mode resonances for silica microspheres. The local electric field enhancement is used to determine the surface enhanced Raman scattering enhancement factors for a molecule. They calculated enhancement factors for dielectric spheres with a refractive index of 1.9 and diameters of 10, 20, and 40 microns for wavelengths across the visible spectrum.
The improved sensitivity obtained while using a whispering-gallery mode resonator is well documented, and in some cases may approach detection of a single molecule. Accordingly, whispering-gallery mode resonator's are well suited for trace analysis. However, optical coupling of the whispering-gallery mode resonator to a spectrometer is reported in the art to require physical contact between the optics of the spectrometer and the resonator, or the optics of the spectrometer must be within the evanescent coupling distance of the whispering-gallery mode resonator, typically a distance of less than one wavelength of the light used as incident electromagnetic radiation.
Accordingly, there is a need for a method and apparatus capable of obtaining spectroscopic measurements using whispering-gallery mode resonators which do not require physical contact between the whispering-gallery mode resonator and the optics of the spectrometer.